1. Field of the Invention
The present invention relates to a demodulator for demodulating a signal containing first and second modulated signals, a disk drive device including the demodulator, and a phase adjustment method of an internal reference wave in the demodulator. More particularly, the present invention relates to a demodulator appropriate for demodulating a minimum shift keying (MSK) modulated signal and a saw-tooth wobble (STW) modulated signal as the first and second modulated signals, a disk drive device, and a phase adjustment method.
2. Description of the Related Art
Demodulation techniques are disclosed in United States Published Patent Application No. 2004/0174800, and Japanese Unexamined Patent Application Publication Nos. 2003-123249, 11-306686, and 2002-74660.
In data recording techniques to record and reproduce digital data, recording media, such as compact disk (CD), a mini-disk (MD), a digital versatile disk (DVD), are used. The optical disk is a generic term referring to recording media made of a thin metal disk protected with plastic. By directing a laser beam onto the disk, a change in a laser beam reflected from the disk is detected to read a signal recorded on the disk.
The optical disks include read-only types, such as CD, CD-ROM (compact disk read-only memory), and DVD-ROM, and read/write types, such as MD, CD-R (compact disk recordable), CD-RW (compact disk−rewritable), DVD-R (DVD recordable), DVD-RW, DVD+RW, and DVD-RAM. In the read/write types, a megneto-optical recording method, a phase change recording method, a color change recording method, and other recording methods are used to record data. The color change recording method, also referred to as write-once method, is considered appropriate for use in data storage because it permits data recording only once. The megneto-optical recording method and the phase change recording method permit data rewriting, thereby finding many applications in the recording of a variety contents including music, video, game, and application programs.
Recently developed high-density optical disks, called Blu-Ray Discs, offer an extremely high data storage capacity.
Guide means for tracking a data track is required to record data onto disks that work in the magneto-optical recording method, the color change recording method, the phase change recording method, or the like. To this end, a groove is formed in a pre-groove process, and the groove or a land (an elevated land portion between grooves in cross section) is tracked in data recording.
Address information needs to be recorded to record data at a predetermined location on a data track. The address information may be sometimes recorded by wobbling the groove.
The track for recording data is formed as a pre-groove, and the sidewall of the pre-groove is wobbled in accordance with the address information.
In this arrangement, addresses can be read from the wobbling information obtained as reflected laser information during recording and reproducing operations. Data is thus recorded to or reproduced from a target location without the need for forming beforehand pit data representing the address on tracks.
The address information arranged as the wobbling groove eliminates the need for arranging address areas discretely on the track and recording an address as pit data. As the address area becomes unnecessary, an amount of actual data recordable is increased accordingly.
Absolute time information (address) represented by the wobbled groove is referred to as the absolute time in pregroove (ATIP), or the address in pregroove (ADIP).
In the case of the Blu-Ray Disk, the groove is wobbled in accordance with a modulated waveform that is modulated in a combination of an MSK modulation and an STW modulation.
The ADIP information, formed using the MSK modulation, the STW modulation, or a combination of both modulations, will be discussed in more detail later. The MSK modulation is one of continuous phase frequency shift keying (FSK) modulations with a modulation index of 0.5.
In the STW modulation, a second harmonic of a wobble fundamental wave is added to or subtracted from the wobble fundamental wave so that a modulated wave, such as a saw-tooth wave, is generated.
The disk drive device of the Blu-Ray Disk, for example, contains an MSK demodulator and STW demodulator to reproduce the ADIP information.
Techniques for demodulating MSK/STW modulated signals, and decoding the ADIP information are disclosed in United State Published Patent Application No. 2004/0174800, and Japanese Unexamined Patent Application Publication Nos. 11-306686, and 2002-74660.
FIG. 24 illustrates a circuit performing the MSK modulation and the STW modulation as a pre-process prior to the decoding of the ADIP information. The circuit performs the MSK modulation and the STW modulation on reflected laser information from the wobbling groove on the disk as the push-pull (P/P) signal (wobble signal). The demodulated signal is then supplied to a subsequent stage ADIP decoder.
The push-pull signal P/P supplied as the wobble signal is fed to an analog-to-digital (A/D) converter 111 and a comparator 112 in an MSK demodulator 110 of FIG. 24.
The comparator 112 binarizes the push-pull signal P/P and supplies a PLL (phase-lock loop) circuit 113 with the binarized push-pull signal P/P. In response to the binarized signal, the PLL circuit 113 generates a clock (hereinafter referred to as a wobble clock WCK) having a wobble frequency of the push-pull signal P/P, namely, of a modulated signal of the wobbling groove.
The wobble clock WCK outputted from the PLL circuit 113 is supplied to a PLL circuit 114 and a delay circuit 116.
The PLL circuit 114 frequency-doubles the wobble clock WCK, thereby generating a master clock MCK. The master clock MCK serves as a sampling clock of the A/D converter 111. The master clock MCK is also used in each of the delay circuit 116 and a counter 117.
The delay circuit 116 gives a predetermined delay to the wobble clock WCK every master clock MCK, and the resulting wobble clock WCK to the counter 117. The delay time is set by a central-processing unit (CPU) 100.
The counter 117 counts the master clock MCK. The counter 117 resets the count thereof at a reset timing in response to the rising edge of the wobble clock WCK applied from the delay circuit 116. More specifically, the counter 117 starts counting the master clock MCK at the reset timing in response to the wobble clock WCK, and outputs the count to a cos (cosine) table 121.
The cos table 121 is a table storing waveform data for an internal reference wave. The data is read in response to the count of the counter 117.
The master clock MCK has a frequency of 23 clocks in one period of the wobble fundamental waveform (wobble clock WCK unit). The counter 117, reset every period of the wobble fundamental waveform, generates counts from 0 through 22.
The cos table 121 stores data TD0-TD22 as a cosine waveform data serving as the internal reference wave. The data TD0-TD22 is successively read in response to the count. In this way, the internal reference wave having the same frequency as the wobble fundamental waveform is generated and then supplied to a multiplier 118.
The A/D converter 111 samples the input push-pull signal P/P in response to the master clock MCK, thereby converting the push-pull signal P/P into digital data (wobble data), and supplying the digital data to the multiplier 118.
The multiplier 118 multiplies the wobble data by the internal reference wave data. The product is then supplied to an accumulator 119. The accumulator 119 is reset in response to the rising timing of the wobble clock WCK from the delay circuit 116. The accumulator 119 is thus reset at the same timing as the reset timing of the counter 117. The accumulator 119 accumulates the products in the wobble fundamental waveform period. For example, the accumulator 119 repeats the accumulation of 23 product samples.
The accumulated value (value summed subsequent to the multiplication) is positive within the wobble fundamental waveform duration in response to the input wobble signal, while being negative within the MSK modulated duration. The positive/negative determiner 120 determines the accumulated value for positive or negative value, thereby resulting in a demodulated signal that identifies an MSK mark and a fundamental wave.
An STW demodulator 130, although not detailed here, is identical in structure to the MSK demodulator 110. The STW demodulator 130 converts the push-pull signal P/P as a modulated signal into digital data, multiplies the digital data by the internal reference wave, and performs a positive/negative determination process on the sum of the products. An STW demodulated signal thus results. In the case of the STW demodulator 130, the internal reference wave is a second harmonic wave of the wobble fundamental waveform. The accumulation of the products is performed in a plurality of wobble durations in which the wobble is STW modulated, rather than in one wobble fundamental waveform.
The wobble signal fluctuates due to crosstalks between adjacent tracks on the disk, an output amplitude difference of the wobble signal between prior to recording and subsequent to recording, and tolerances in the quality of the disk. The use of an automatic gain control (AGC) circuit and the limiting of the amplitude of the wobble signal are contemplated to control variations in the amplitude of the wobble signal, as disclosed in Japanese Unexamined Patent Application Publication Nos. 11-306686 and 2002-74660. The wobble signal is subject to disturbance not only in amplitude but also in time axis (phase).
As will be discussed later, to control phase variations, a reference signal is used to adjust a phase of the internal reference wave for the detection of the demodulator in the STW modulated signal of the wobble signal in an ADIP format of the Blu-Ray Disk. When the amplitude variations are large due to external disturbance, demodulation becomes difficult.
No function to detect a phase variation due to external disturbance is originally available in the demodulation of the MSK modulated signal. The demodulation of the MSK modulated signal is difficult when the variations in phase due to external disturbance become large.
As shown in FIG. 24, the wobble signal (push-pull signal P/P) is accumulated within a predetermined duration of time after being multiplied by the internal reference wave. After the accumulated value is subjected to the positive/negative determination process, an MSK demodulated signal and an STW demodulated signal are obtained.
FIG. 25A illustrates a wobble signal and an MSK demodulated signal as the accumulated output of the accumulator 119. FIG. 25B illustrates the waveform of FIG. 25A in a time scale contracted form.
As phase variations due to external disturbance become large, the MSK demodulated signal waveform (accumulated value) periodically varies in amplitude. MSK demodulation becomes difficult where the amplitude of the signal becomes small. The MSK demodulation becomes difficult particularly when a disk large in a beat noise level is replayed, or when a disk having a focus offset or a large media inclination is replayed.